I attended a workshop held by the Historical Commission of Texas about restoring and cleaning Tombstones and all ways to record one etc. For Lichen (moss) they say to gently brush it off with a very soft brush or cloth. Treat the surface like a paintjob on a car and it wont' damage the surface. They said the moss is often better left on than the process to take it off but if you must then use a cleaning material from a hardware or farm shop called Orvus. it is also sold in janitorial supplies. Also never put clorox or any bleaching material on a stone as it causes chemical reactions and will turn yellow in just a few years- never can be removed. The stones also must breathe and cannot be sealed or waxed in any fashion or the ground moisture will be trapped inside. Susan in Texas